1. Field of the Invention
The present invention generally relates to the measurement of formation dip angle relative to a wellbore. More particularly, the present invention relates to a method for determining the dip angle using a virtually steered induction tool.
2. Description of the Related Art
The basic principles and techniques for electromagnetic logging for earth formations are well known. Induction logging to determine the resistivity (or its inverse, conductivity) of earth formations adjacent a borehole, for example, has long been a standard and important technique in the search for and recovery of subterranean petroleum deposits. In brief, the measurements are made by inducing electrical eddy currents to flow in the formations in response to an AC transmitter signal, and measuring the appropriate characteristics of a receiver signal generated by the formation eddy currents. The formation properties identified by these signals are then recorded in a log at the surface as a function of the depth of the tool in the borehole.
Subterranean formations of interest for oil well drilling typically exist in the form of a series relatively thin beds each having different lithological characteristics, and hence, different resistivities. Induction logging is generally intended to identify the resistivity of the various beds. However, it may also be used to measure formation xe2x80x9cdipxe2x80x9d.
Wellbores are generally not perpendicular to formation beds. The angle between the axis of the well bore and the orientation of the formation beds (as represented by the normal vector) has two components. These components are the dip angle and the strike angle. The dip angle is the angle between the wellbore axis and the normal vector for the formation bed. The strike angle is the direction in which the wellbores axis xe2x80x9cleans away fromxe2x80x9d the normal vector. These will be defined more rigorously in the detailed description.
The determination of the dip angle along the length of the well plays an important role in the evaluation of potential hydrocarbon reservoirs and in the identification of geological structures in the vicinity of the well. Such structural and stratigraphic information is crucial for the exploration, production, and development of a reservoir.
Currently, there are several ways of measuring formation dip: (1) electrode (pad) devices, such as those taught in U.S. Pat. No. 3,060,373, filed June 1959 by H. Doll, and U.S. Pat. No. 4,251,773, filed June 1978 by M. Calliau et al.; and (2) Electric imaging devices. Both require that good electrical contact be maintained during the logging process. Under adverse conditions, such as in oil based mud drilling, or when the borehole is highly rugose, good electrical contact between the pads and the formation is difficult to maintain.
U.S. Pat. No. 4,857,852, filed April 1988 by R. Kleinberg et al., discloses a microinduction dipmeter to overcome the high resistivity of oil-based mud. Kleinberg replaces the electrodes of the resistivity dipmeter with microinduction coil transmitter and receivers. Although it can be used in oil based mud, the high operation frequency (20-30 MHz) means it provides a rather limited depth of investigation. As such, the dipmeter will be adversely affected by unpropitious borehole conditions such as borehole invasion and borehole rugosity.
One way to combat these disadvantages would be the use of an electromagnetic induction dipmeter. Such a dipmeter would preferably operate in those wells where the use of conductive mud is not viable. Furthermore, such an induction dipmeter should have a depth of investigation deep enough to minimize the adverse effects of the borehole geometry and the invasion zone surrounding it (xe2x80x9cborehole effectxe2x80x9d).
An induction dipmeter was first suggested by Moran and Gianzero in xe2x80x9cEffects of Formation Anisotropy on Resistivity Logging Measurementsxe2x80x9d Geophysics, Vol. 44, No. 7, p. 1266 (1979). This dipmeter was deemed not feasible because it possessed a sensitivity to the borehole effect because of the small transmitter-receiver spacing. To overcome this limitation, U.S. Pat. No. 5,115,198, filed September 1989 by Gianzero and Su, proposed a pulsed electromagnetic dipmeter that employs coils with finite spacing. However, even though pulsing the dipmeter does remove the requirement for zero transmitter-receiver spacing, the use of the time-dependent transient signals unduly complicates the design and operation of the tool compared with conventional induction tools running in Continuous Wave (CW).
These attempts to provide a commercial induction dipmeter have thus far not succeeded. An economical yet accurate new technique is therefore needed.
Accordingly, there is disclosed herein a method of using the various cross-coupling measurements generated by a triad induction tool to identify the formation strike and dip angles. The method virtually rotates the transmitters and receivers, calculates derivatives of the couplings and the dependence of those derivatives on the rotation angle, and based on this dependence, calculates the dip angle of the formation. These calculations can be performed in real time. In one embodiment, the method includes: (1) measuring a magnetic coupling between transmitter coils and receiver coils of a tool in the borehole; (2) obtaining from the measured coupling a strike angle between the tool and the formation; (3) applying a rotational transformation to the coupling measurements to correct for the strike angle; and (4) applying a predetermined set of rotational transformations to the coupling to determine coupling term values as a function of rotation angle. The derivative of the coupling term values with respect to position is postulated to have a functional form in which the dip angle is one of the parameters. A least-squares curve fit or a Hough transform may be used to identify the dip angle.
The disclosed method may provide the following advantages in determining the formation dip angle: (1) As an induction apparatus, the disclosed method can be applied in situations where the condition are not favorable for the focused current pad dipmeters, e.g., in wells drilled with oil based mud or when the wellbore has high rugosity. (2) Only the real part of the voltages need be measured, so measurement of the unstable imaginary signal may be avoided. (3) Since the derivatives of the signals are used, the current method may have a greatly reduced borehole effect. (4) The disclosed method has a deeper depth of investigation than the microinduction pad dipmeter and hence provides a direct measurement of the regional dip that is less vulnerable to adverse borehole conditions.